This invention relates to a remote plasma deposited fluoropolymer film, and more particularly to a high density remote plasma deposited fluoropolymer film. The present invention also relates to a fluoropolymer film formed on the front face of a thermal ink jet printhead and a method for forming the fluoropolymer film, particularly on the front face of a thermal ink jet printhead.
In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359, to Ayata et al. At one end, these channels communicate with a relatively small ink supply chamber. At the opposite end, the channels have an opening referred to as a nozzle. A thermal energy generator, for example a resistor, is located in each of the channels a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize ink in the respective channels and thereby form an ink bubble. As the bubble grows, the ink bulges from the nozzle, but it is contained by the surface tension on the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle, and bubble starts to move towards the collapsing bubble causing a volumetric contraction of the ink at the nozzle resulting in the separation of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides momentum and velocity towards a recording medium, such as paper.
The specific details of the separation of the ink from its physical surroundings, the ink channel, and its orifice determine to a large extent the direction in which the ink will travel to the paper and thus where the mark on the paper will be made. Any microscopic irregularity that would effect the isotropy of this ink/orifice separation process will usually cause the ink to travel in an uncontrolled and unintended direction, that is, for example, not orthogonal to the plane defined by the front face. This results in poor quality of the images and text that are printed on the paper. Such irregularities include pools of ink that collect around the orifice from previous jet firing. For example, FIG. 1 demonstrates drop misdirectionality of an ink jet printhead. The amount of spot misplacement is a function of the off-axis velocity multiplied by the print distance divided by the nominal drop velocity. Thus, if any of these factors are affected, for example by microscopic irregularities at the ink orifice, the ink droplets will be misdirected as indicated in FIG. 1.
Microscopic irregularities can be avoided by providing a coating on the exit orifice that repels the ink that is used for the printing process. To avoid or minimize ink drop deflection problems that can lead to subsequently printed images of poor quality, the front face of ink jet devices may be coated, particularly around the nozzles, with one or more ink repellent layers.
Various ink repellent layers coated on the front face of a thermal ink jet printhead are known in the art. Methods for coating the front face include spraying or dip coating hydrophobic liquids onto the front face of the printhead device or coating a material onto an intermediate substrate and then transferring the coated material onto the front face of the device using some combination of pressure and heat. Material can also be applied to the front face using vapor deposition methods such as chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering or thermal evaporation.
U.S. Pat. No. 5,043,747 to Ebisawa et al. is directed to a polymer derivative compound of 1,3- or 1,4-bis(hexafluoroisopropyl)benzene, or 2,2-bisphenylhexafluoropropane used as the front face coating material and applied via intermediate substrate transfer.
JP-A-63-122560 (JP-A stands for a published unexamined Japanese Patent Application) discloses an ink repellent layer coated onto the surface of an elastic member and then subsequently transferred onto the surface of the ink jet device at the peripheral portion of the nozzles. JP-A-63-122557 discloses applying an ink repellent layer on a printhead device by dipping the printhead into an ink repellent agent while gas is jetted out through the openings. JP-A-63-122550, JP-A-63-122559, and JP-A-56-98569 disclose ink repellent agents containing fluorine atoms.
Plasma deposition, or glow discharge as it is often referred, is preferred due to its ease in allowing large batches of substrates, such as die modules, to be treated simultaneously, thus enabling high throughput. Uniformity of coating from device-to-device and batch-to-batch is also well controlled due to the relative sophistication of state-of-the-art plasma processing equipment.
Plasma treatment (also referred to as plasma surface modification) or plasma deposition of thin films may generally be performed in either of two processing setups: direct or remote. With direct plasma processing, film treatment or growth is within the plasma region. A typical apparatus as shown in FIG. 2 utilizes a parallel plate type reactor with the substrate 1 placed between electrodes 2 and 3 in a vacuum chamber 4 and resting on the lower electrode 2 and in contact with the plasma 5. With remote deposition, the substrates are removed from the plasma region. Reactive species created in the plasma must be transported to the substrate to deposit thereon. The effect of chamber pressure in determining the mean free path of these species, i.e., how far they can travel, is significant. The substrate may also be independently biased relative to the plasma to allow for control of energetic ion interaction with the film.
In addition to the relative location of the substrates during film treatment of growth, the nature of the plasma source is critical in determining the chemistry of species which interact with the film or substrate. Radio frequency (13.56 MHz) and direct current generated plasmas typically result in an ion and electron density of about 10.sup.10 /cm.sup.3 and neutral radical density of about 10.sup.14 /cm.sup.3. High density plasmas, such as those produced using microwave electron cyclotron resonance, inductive coupling and helicon wave generators result in electron/ion dominated plasmas with densities near 3.times.10.sup.11 /cm.sup.3. These high densities can offer advantages with regard to subsequent material properties and processing times. The preparation of plasma deposited fluoropolymer films has been a topic of scientific experimentation for many years and is extensively summarized in Plasma Deposition, Treatment and Etching of Polymers, edited by R. D'Agostino, Academic Press, 1990, Chapter 2. In general, the higher the fluorine to carbon (F/C) ratio and the more CF.sub.2 and CF.sub.3 type bonding, as compared with CF type bonding, the more hydrophobic the material and more effective the material is as an ink repellent front face coating. A --CF.sub.2 -- bonding structure, as is found in polytetrafluoroethylene (PTFE), i.e. Teflon.RTM. (F/C equals 2), results in a low surface energy and makes the layer highly hydrophobic. Pure PTFE as it exists in its typical bulk film form however, cannot be plasma deposited. The use of such fluoropolymer films for thermal ink jet device front face coatings has been described in copending patent application Ser. No. 08/369,439. This application describes the use of a radio frequency generated plasma using various fluorocarbon gases to form a deposited fluoropolymer film on a substrate. Because the film is deposited as a bulk layer, the nature of the substrate is relatively unimportant in determining the ultimate surface energy properties of the fluoropolymer film and therefore the nature of the substrate is restricted only by the ability to obtain acceptable adhesion of the fluoropolymer film. This copending application describes the use of a parallel plate type system as in FIG. 2 for depositing such films. The limitation imposed by this process resides in the required chamber pressure necessary to sustain the radio frequency (rf) plasma and the reactive species which can be created due to the relatively low ion and electron density in the plasma. A chamber pressure of nearly 100 mTorr is typically required to ensure the stable operation of the plasma. At these pressures however, the mean free path of the reactive species created in the plasma is relatively short (typically &lt;1 mm) leading to primarily gas phase collisions and polymerization. These polymerized products, although rich in fluorine content, deposit as a low density, poorly cross-linked material resulting in a film of poor mechanical properties.
In contrast, instead of depositing a bulk fluoropolymer film, a fluoropolymer layer can be created by modifying the surface of a substrate material using plasma processing. Whether surface modification of deposition of a fluoropolymer film occurs depends on the nature of the fluorocarbon source gas and other processing parameters such as substrate, temperature, chamber pressure and applied power to the plasma. Such surface modification is discussed in Plasma Surface Modification and Plasma Polymerization, in N. Inagaki, Technomic Publishing Company, Inc., 1996, Chapter 4.
However, U.S. Pat. No. 5,073,785 to Jansen et al., which is incorporated by reference herein in its entirety, discloses a process for minimizing or avoiding ink drop deflection in ink jet devices that comprises coating the front face of ink jet head components with an amorphous or diamond-like carbon layer. The amorphous or diamond-like carbon layer is subsequently fluorinated with a fluorine-containing gas by plasma enhanced chemical vapor deposition (PECVD) to render its surface stable and hydrophobic. Such a treatment does not deposit a coating, but merely modifies the physical and chemical properties of the exposed surface by the saturation of dangling bonds.
Further Jansen et al. discloses that fluorine can be incorporated into the material when PECVD is used as a deposition technique for the diamond-like carbon films once again leading to bulk deposited fluoropolymer films. Jansen et al. discloses that fluorinated gases can be used as precursor gases, but often require the presence of hydrogen. Jansen et al. does not disclose the types of fluorinated gases or the amount of hydrogen that may be used.
However, using the process of Jansen et al., only a limited concentration of fluorine can be achieved and the nature of its bonding is primarily CF instead of CF.sub.2 or CF.sub.3 because fluorine atoms are simply replacing hydrogen atoms on the surface or passivating unsaturated bonds. This is a result of the processing method employed by Jansen et al, namely direct plasma processing using a radio frequency generated plasma. Because the substrates are positioned within the plasma, all reactive species created are able to interact with the substrate in the surface modification reaction. The low electron density rf plasma creates primarily atomic fluorine atoms that accomplish this modification, resulting in the obtained surface stoichiometry. The lack of fluorine limits the level of hydrophobicity that can be obtained. A typical F/C ration for fluorinated diamond-like carbon is about 0.3, with 80% of the fluorine incorporated as CF type bonds and only 20% as CF.sub.2 and CF.sub.3 type bonds.
However, these plasma processes do not provide a material that has sufficient hydrophobicity and mechanical durability. Accordingly, what is desired is a plasma processing method where a fluoropolymer layer, i.e., a surface modification of the substrate material is provided where a high concentration of CF.sub.2 and CF.sub.3 type bonding groups are incorporated into the matrix of the substrate material. This minimizes the deposition of a mechanically soft fluoropolymer film and provides a highly ink repellent film with excellent mechanical durability suitable for advanced thermal ink jet front face coating applications.